Filter element with coating for surface filtration

ABSTRACT

An inherently stable, flow-porous filter element ( 2 ) having a porous coating ( 10 ) for surface filtration on its afflux surface, said coating ( 10 ) comprising: 
     (a) a main component of particles ( 12 ) and/or fibers ( 22 );    (b) an inorganic binder component ( 14; 24 ) binding the particles ( 12 ) and/or the fibers ( 22 ) of the main component to each other;    (c) an adhesive ( 16; 26 ) binding the porous coating ( 10 ) to the remainder of the filter element;    (d) and an anti-adhesion component ( 18; 28 ) mitigating the adhesion of filtered out material to the porous coating and facilitating cleaning of the filter element.

The invention relates to an inherently stable, flow-porous filterelement having a porous coating for surface filtration on its affluxsurface.

Such filter elements are known with a number of coatings of differentconstruction.

It is the object of the invention to make available a filter elementwith a surface filtration coating that is useful also for increasedtemperatures and/or increased resistance to chemical attack.

To meet this object, the coating of the filter element comprises:

-   (a) a main component of particles and/or fibers;-   (b) an inorganic binder component binding the particles and/or    fibers of the main component to each other;-   (c) an adhesive binding the porous coating to the remainder of the    filter element;-   (d) and an anti-adhesion component mitigating the adhesion of    filtered out material to the porous coating and facilitating    cleaning of the filter element.

The binding of the coating to the remainder of the filter element neednot be effected exclusively by the adhesive, but the adhesive has aconsiderable share in effecting this binding. On the other hand, theadhesive may participate in addition also in binding the particlesand/or fibers of the main component to each other.

Preferred particles for the main component are kieselguhr particles,zeolite particles, polyvinyl pyrrolidone particles and mixtures thereof;among these, kieselguhr particles are particularly preferred. Preferredfibers for the main component are ceramic fibers. The preferredsubstances mentioned do not have to form 100% of the main component; itis sufficient when the main component contains one or more of thesubstances referred to as preferred ones to a considerable share. On theother hand, it is preferred that the main component consists in essenceof only one of the preferred substances mentioned or mixtures thereof.As an alternative, however, it is preferred as well to use a mixture ofceramic fibers and one type of particles referred to as preferred orseveral types of particles referred to as preferred.

The binder component may comprise or consist of water glass. However, itis preferred that the binder component is not water glass. The bindercomponent preferably is sol-based. The binder component preferablycomprises crosslinked SiO₂ or crosslinked SiO₂ derivative or crosslinkedTiO₂ or crosslinked TiO₂ derivative or crosslinked ZrO₂ or crosslinkedZrO₂ derivative or a mixture of several of the afore-mentionedsubstances, Crosslinked SiO₂ is very much preferred. Also as regards thebinder component, it is not necessary for the same to consist to 100% ofone of the substances referred to as preferred or of a mixture ofseveral of these substances. However, it is preferred that the bindercomponent consists in essence of one of the substances referred to aspreferred or of a mixture of several of these substances. SiO₂derivative is understood to be a substance which, apart from the siliconatoms and the oxygen atoms, still contains some organic material. Thesame holds analogously for TiO₂ and ZrO₂.

Preferably, the adhesive is an organic dispersion adhesive. Aqueousdispersions are particularly preferred.

Preferably, the anti-adhesion component is polytetrafluoroethylene—inthe following briefly referred to as PTFE—and/or silicone. Very muchpreferred is PTFE. Moreover, the case very much preferred is that theanti-adhesion component in essence consists of PTFE only. A particularlyexpedient embodiment of the invention provides for the application ofthe coating pursuant to the sol-gel process.

A further expedient embodiment of the invention consists in that thecoating (in essence) contains kieselguhr (only) as main component and(in essence) PTFE (only) as anti-adhesion component in a weight ratio of1:1 to 1;2.

The main body of the filter element, i.e. the basic structure of thefilter element located underneath the coating, preferably is composed ofsintered plastics particles. Polyethylene particles are particularlyfavorable.

The modification of the filter element according to the invention inwhich particles are present as main component of the coating, can beused in particularly favorable manner for liquid filtration or forseparating oil mists from a gas stream. The filtration of coolinglubricant is envisaged in particular in this regard; cooling lubricantsare oily liquids or emulsions of lubricating substances in particular inwater that are used in mechanical or abrasive or cutting or othermaterial-removing (e.g. spark erosion) working (machining) ofworkpieces. The cooling lubricant as a rule is pumped in circulatingmanner and contains, when leaving the place of workpiece machining,abraded, cut or removed particles of the machined workpiece. Theseparticles need to be filtered out before the cooling lubricant againreaches the workpiece in the circulation. Preferably, the filtration isperformed by means of the filter element according to the inventionwhich is simply immersed into a bath of the cooling lubricant. Thefiltered out foreign particles adhere to the coating on the outersurface of the filter element the filtered cooling lubricant is suckedoff from inside of the filter element. For periodic cleaning of thefilter element, the pump circulation is briefly interrupted at suitableintervals in time and replaced by a brief liquid stream of cleanedcooling lubricant in the opposite direction, so that the filtered outforeign particles accumulated on the outside of the coating drop downinto the cooling lubricant bath and may be removed from there e.g. bymeans of a sludge discharge means.

On the basis of the teaching of the invention it is possible to producefilter elements which in essence completely filter out foreign particlesof a size of more than 5 μm, more preferably more than 3 μm, from aliquid. In case of cooling lubricant, the effect achieved is that themachining accuracy of the workpiece is very high as there is extremelyclean cooling lubricant flowing to the machining site. It is possible toproduce filter elements according to the invention for gas filtrationwhich filter out foreign particles of a size of more than 3 μm, morepreferably of a size of more than 1 μm, in essence completely.

The variant of the filter element according to the invention in whichfibers are present as main component of the coating (to a by farpredominant extent or in essence as sole component) can be used inparticularly expedient manner for gas filtration.

All in all, the filter element according to the invention can be used inparticularly advantageous manner in situations in which resistance toincreased temperatures and/or resistance to chemical attack isimportant. The filter element may be designed to resist a temperature ofpermanent use of up to 180° C., or it may be designed to resist atemperature of permanent use of up to 150° C., or it may be designed toresist a temperature of permanent use of up to 120° C. Chemical attackoccurs e.g. in case of the afore-mentioned cooling lubricant. Anotherapplication involving chemical attack is the filtering of combustionexhaust gas. As examples for applications involving temperatureresistance, there can be named the filtering of combustion exhaust gas,filtering for product separation from a gas stream (e.g. spray drier,drier in foodstuff industry) and materials recycling from a gas stream(e.g. catalyst in fluidized bed reactors).

The invention and preferred developments of the invention will beexplained in more detail in the following by way of embodiments.

FIG. 1 shows a sectional view of a fragment of a filter element;

FIG. 2 shows a sectional view of a fragment of a filter elementaccording to a another embodiment,

In the filter element 2 shown as a fragment in FIG. 1, the maincomponent consists of polyethylene particles 4 that are sinteredtogether at their contact locations 6. The pores 8 between the particles4 have an average size of e.g. 10 to 60 μm.

The coating 10 of the filter element in essence consists of kieselguhrparticles 12 that are bonded to each other via crosslinked SiO₂ 14 and,at their contact locations with polyethylene particles 4, are bonded tothe main body by means of an organic dispersion adhesive 16, and of PTFEparticles 18. The kieselguhr particles 12 are so small that the poresbetween them in the average have a size of less than 3 μm. In producingthe filter element 2 of FIG. 1, the main body is produced first andthereafter the coating is applied to the afflux surface of the sameeither with the consistence of a rather thin liquid or with theconsistence of a rather thick liquid to a pasty consistence, e.g. byspraying, rolling on, brush application, etc. The coating mass to beapplied in essence consists of kieselguhr particles, an SiO₂ sol, PTFEparticles, organic dispersion adhesive, water and a small amount oftenside, wetting aid or foam prevention means. All of these componentsare available on the market without any problem. The coating mass isemulsion-stable. Upon application of the coating mass, there is asol-gel reaction, using as a rule a slightly increased temperature, andafter evaporation of all water, the coating is formed in the solidaggregate state. During the sol-gel reaction and evaporation of thewater, crosslinking of the SiO₂ phase and thus binding of the kieselguhrparticles to each other takes place.

The weight ratio of kieselguhr to PTFE is in the range from 1:1 to 1:2for obtaining particularly good results.

The filter element 2 shown in a fragment in FIG. 2 differs from thefilter element according to FIG. 1 by a different coating 10. In thiscase, the finished coating 10 in essence consists of ceramic fibers 22,crosslinked SiO₂ 24, organic dispersion adhesive 26 and PTFE particles28. Kieselguhr particles 12 in a smaller amount than in FIG. 1 arepresent as well. As regards the coating mass in the state to be applied,the statements (additional components, consistence etc.) made inconnection with the embodiment according to FIG. 1 are applicable again.

The pores between the ceramic fibers 22 have substantially the same sizeas the pores between the kieselguhr particles 12 in the embodimentaccording to FIG. 1. The ceramic fibers 22 typically have a diameter ofa few μm and a length of 10 to 30 μm; chemically, they consist typicallyin essence of SiO₂/Al₂O₃.

Kieselguhr is a common designation for a material consisting typicallyto 70 to 90% of amorphous silicic acid and to 3 to 12% of water.

In case of silicone as anti-adhesion component, the drawing figureswould have a substantially unchanged appearance. In case of water glassas inorganic binder component, the drawing figures would have asubstantially unchanged appearance.

1. An inherently stable, flow-porous filter element (2) having a porouscoating (10) for surface filtration on its afflux surface, said coating(10) comprising: (a) a main component of particles (12) and/or fibers(22); (b) an inorganic binder component (14; 24) binding the particles(12) and/or the fibers (22) of the main component to each other; (c) anadhesive (16; 26) binding the porous coating (10) to the remainder ofthe filter element; and (d) an anti-adhesion component (18; 28)mitigating the adhesion of filtered out material to the porous coatingand facilitating cleaning of the filter element.
 2. A filter elementaccording to claim 1, characterized in that the main component comprisesone material of the group consisting of kieselguhr particles (12),zeolite particles, polyvinyl pyrrolidone particles and mixtures thereof.3. A filter element according to claim 2, characterized in that the maincomponent comprises kieselguhr particles (12).
 4. A filter elementaccording to claim 1, characterized in that the main component comprisesceramic fibers (22).
 5. A filter element according to claim 1,characterized in that the binder component comprises one material of thegroup consisting of crosslinked SiO₂ (14; 24), crosslinked SiO₂derivative, crosslinked TiO₂ derivative, crosslinked ZrO₂, crosslinkedZrO₂ derivative and mixtures thereof.
 6. A filter element according toclaim 5, characterized in that the binder component comprisescrosslinked SiO₂ (14; 24).
 7. A filter element according to claim 1,characterized in that the adhesive is an organic dispersion adhesive(16; 26).
 8. A filter element according to claim 1, characterized inthat the anti-adhesion component is polytetrafluorothylene (PTFE) (18;28) and/or silicone.
 9. A filter element according to claim 1,characterized in that the coating (10) is applied in accordance with thesol-gel process.
 10. A filter element according to claim 1,characterized in that the coating (10) contains kieselguhr (12) as maincomponent and PTFE (18; 28) as anti-adhesion component in a weight ratioof 1:1 to 1:2.
 11. A filter element according to claim 1, characterizedin that the main body of the filter element (2) is composed of sinteredplastics particles (4).
 12. The use of the filter element according toclaim 1, comprising particles (12) as main component of the coating(10), for liquid filtration or for oil mist separation.
 13. The useaccording to claim 12 for filtration of cooling lubricant.
 14. The useaccording to claim 12, wherein foreign particles of a size of more than5 μm are filtered out in essence completely.
 15. The use of the filterelement according to claim 1, comprising fibers (22) as main componentof the coating (10), for gas filtration.
 16. The use according to claim15, wherein foreign particles of a size of more than 2 μm are filteredout in essence completely.